In an article by Th. Pfeiffer and G. Veith, "40 GHz pulse generation using a widely tunable all-polarization preserving erbium fibre ring laser", Electronic Letters, October 1993, pp. 1849-1850, an actively mode-locked fiber ring laser for generating optical pulses is described. The fiber ring laser contains an erbium-doped fiber, a pump-light source, an optical isolator, an optical filter, and a Mach-Zehnder modulator. To generate the optical pulses, the Mach-Zehnder modulator is driven by an AC voltage source. When a voltage is applied to the Mach-Zehnder modulator, the refractive index in the Mach-Zehnder arms will change briefly, so that interference phenomena will result in the signal light being amplitude-modulated. If the voltage is an alternating voltage, optical pulses will thus be developed at the output of the fiber ring laser. The AC voltage source operates in the GHz range and must be adjusted very accurately to the round-trip frequency of the light in the ring resonator. In addition, operation at high voltages is necessary, which requires the use of electrical high-frequency amplifiers.